The present invention relates to a single crystal of an oxide garnet or, more particularly, to a novel single crystal of an oxide garnet useful as a material of magneto-optical devices such as optical isolators, optical switches and the like exhibiting improved behavior of light absorption at the working wavelengths.
It is an established prior art that magneto-optical devices such as optical isolators and the like are prepared from a single crystal layer of a bismuth-substituted rare earth-iron garnet grown on a substrate single crystal by the liquid-phase epitaxial method. There is an unavoidable problem in this liquid-phase epitaxial method that the garnet single crystal is heavily contaminated with lead or platinum originating in the lead oxide used as a flux of the oxide melt and the platinum-made crucible, respectively. When such a contaminated epitaxial garnet single crystal is used as the magneto-optical element, absorption of light is greatly increased at the working wavelengths of 0.8 .mu.m, 1.3 .mu.m and 1.55 .mu.m to cause an increase in the insertion loss.
A remedial measure for the above mentioned problem is proposed in Preprint for the 11th Conference of Japan Association of Applied Magnetics, Nov. 1987, 2C-10, page 137, according to which trace amounts of divalent or tetravalent metallic ions, e.g., Ca.sup.2+, Mg.sup.2+ and Ti.sup.4+, are added to the oxide garnet single crystal of this type. This method, however, does not provide a complete solution of the problem because the addition of the above mentioned metallic ions to the melt for the epitaxy necessarily causes changes in the composition of the epitaxially grown layer in the course of the growth so that the uniformity in the performance of the epitaxial garnet layer throughout the thickness can hardly be ensured since the epitaxial layer is usually required to have a thickness as large as 50 .mu.m or larger.